In many optical systems small details are to be resolved by a light beam. For example, in an optical disk drive the data is read out by focusing a beam of wavelength λ and numerical aperture NA onto the data layer and measuring the amount of light that is reflected back into the aperture of the lens. The same principle is applied in scanning microscopes. The smallest resolvable detail in these systems is of the order λ/NA. In conventional imaging systems, such as an optical lithography apparatus, the NA of the objective lens determines the resolution in much the same way as for a scanning microscope. For all these systems it holds that for small to moderate NA the polarization of the light beam does not play a very significant role. However, for large NA the polarization state is highly relevant for the resolving properties of the system. Conventionally, the polarization is taken to be uniform across the pupil of the system, and either linear or circular. A non-uniform polarization state alters the distribution of light close to the focal point. For example, a beam with a radially oriented linear polarization across the pupil is reported to result in a relatively narrow focal spot (cf. R. Dorn, S. Quabis, and G. Leuchs, Sharper focus for a radially polarized light beam, Physical Review Letters, Volume 91, 233901, 2003). When such a radially polarized beam is further modified by blocking the central part of the pupil (so-called apodization) the polarization state across the focal spot is substantially linear and oriented along the optical axis of the system. This stands in contrast to low NA imaging with a uniform linear polarization where the polarization state across the focal spot is substantially linear and oriented perpendicular to the optical axis. Thus, providing radially polarized light beams within an optical system allows for novel kinds of imaging.
According to prior art, radially polarized beams are difficult to produce. For example, it is required to modify the laser, or to introduce segmented wave plates and clean-up optical filters, or to use complicated computer generated diffractive elements.
An object of the present invention is to provide a light beam with a desired linear polarization across the pupil with simple means, and particularly with a radially oriented linear polarization across the pupil.